The invention relates to a phase control loop with a voltage-controlled oscillator and a control substitution (or substitute clock change-over) circuit, that interrupts the control loop in case the reference signal or command fails, and allows the voltage-controlled oscillator or VCO to proceed with a nominal unvarying control voltage. Such a phase control loop is, for example, known from the German Patent Application DE 33 24 919 A1 and DE 34 41 226 A1.
In a phase control loop as disclosed in the German Patent Application DE 33 24 919 the "substitute clock change-over" circuit comprises a clock generator having a very high frequency compared to the reference frequency (command frequency) of the control loop, a circuit for monitoring the reference frequency, an up-down counter and a switching matrix. Despite this circuit complexity it may occur in high-speed control loops that the voltage-controlled oscillator continues to operate at a frequency that considerably differs from the frequency it furnished prior to the failure of the reference frequency. To improve such a control loop, the "substitute clock change-over" circuit according to DE 34 41 226 is extended by a computer, an integrator and a threshold detector.
It is clear from the above description that, as used herein, "substitute clock change-over" does not refer to substitution of a different clock oscillator, but rather to substitution of a different clock control circuit to allow the voltage-controlled oscillator to continue to operate after the command (reference frequency) has failed. Thus "substitute clock change-over circuit" refers to what is actually a control substitution circuit.